1. Field of the Invention
The present invention relates to an apparatus for pulling a single crystal and, more particularly, to an apparatus for pulling a single crystal used for pulling a single crystal of silicon or the like from a melt of a material for forming a crystal by a pulling method such as the Czochralski method (hereinafter, referred to as the CZ method).
2. Description of the Relevant Art
At present, the majority of silicon single crystals used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like have been pulled by the CZ method. FIG. 3 is a diagrammatic sectional view of a conventional apparatus for pulling a single crystal used for the CZ method, and in the Figure, reference numeral 21 represents a crucible.
The crucible 21 comprises a bottomed cylindrical quartz crucible 21a and a bottomed cylindrical graphite crucible 21b fitted on the outer side of the quartz crucible 21a. The crucible 21 is supported with a support shaft 28 which rotates in the direction shown by the arrow A in the Figure at a prescribed speed. A heater 22 of a resistance heating type and a heat insulating mold 27 arranged around the heater 22 are concentrically arranged around the crucible 21. The crucible 21 is charged with a melt 23 of a material for forming a crystal which is melted by the heater 22. On the central axis of the crucible 21, a pulling shaft 24 made of a pulling rod or wire is suspended, and at the front thereof, a seed crystal 35 is held by a holder 24a. These members are arranged within a water cooled type apparatus body 29 wherein pressure can be controlled.
A window 29a is formed in the middle portion in the height direction of the apparatus body 29. Above the window 29a in a slanting direction, a one-dimensional CCD camera 11 is arranged in such a position that a single crystal 36 during pulling can be seen through the window 29a, and the one-dimensional CCD camera 11 is connected to a diameter measuring means 8 of the single crystal 36. The diameter measuring means 8 is connected to a crucible ascent speed calculating means 9, which captures an input value of the pulled single crystal 36 from the diameter measuring means 8 and information on the pulled length from a pulling means 5 of the pulling shaft 24 to calculate a decrease in volume of the melt within the crucible 21, so as to calculate an ascent speed of the crucible 21 based on the decrease in the melt volume.
A method for pulling a single crystal 36 using the above apparatus for pulling a single crystal is described with reference to FIGS. 3 and 4(a)-(d). FIGS. 4(a)-4(d) are partial enlarged front views diagrammatically showing a seed crystal and the vicinity thereof in part of the steps in pulling a single crystal.
Although it is not shown in FIGS. 4(a)-(d), the pressure within the apparatus body 29 is reduced, and an inert gas is introduced into the apparatus body 29 so as to make an inert gas atmosphere under reduced pressure therein. Then, the material for forming a crystal is melted by the heater 22 and is maintained for a period of time so as to sufficiently release gas contained in the melt 23. While the pulling shaft 24 is rotated on the same axis in the reverse direction of the support shaft 28 at a prescribed speed, the seed crystal 35 held by the holder 24a is caused to descend and is brought into contact with the melt 23. After the front portion of the seed crystal 35 is partially melted into the melt 23, the single crystal 36 begins to be pulled from the melt (the seeding step) (FIG. 4(a)).
In making a crystal grow at the front of the seed crystal 35, the pulling shaft 24 is pulled at a higher speed than the below-described formation speed of a main body 36c, and the crystal is narrowed to have a prescribed diameter, leading to the formation of a neck 36a (the necking step) (FIG. 4(b)). By slowing down the pulling speed of the pulling shaft 24 (hereinafter, simply referred to as the pulling speed), the neck 36a is made to grow to have a prescribed diameter, leading to the formation of a shoulder 36b (the shoulder formation step) (FIG. 4(c)). By pulling the pulling shaft 24 at a fixed rate, the main body 36c having a uniform diameter and a prescribed length is formed (the main body formation step) (FIG. 4(d)).
Furthermore, although it is not shown in FIG. 4, in order to prevent induction of high density dislocation to the single crystal 36 by a steep temperature gradient at the end, the diameter of the single crystal 36 is gradually decreased and the temperature of the whole single crystal 36 is gradually lowered, leading to the formation of an end-cone. Then, the single crystal 36 is separated from the melt 23. After the above steps, cooling the single crystal 36 leads to the completion of pulling the single crystal 36.
In pulling the single crystal 36, in order to obtain a good quality single crystal 36, it is necessary to drive the support shaft 28 using a motor 10 for crucible elevating to move up the crucible 21 up to control the position of the crucible 21 so that the melt surface is always kept in a fixed position with respect to the heater 22. In the crucible ascent control, a method wherein the amount of ascent of the crucible 21 is calculated by calculating a decrease in volume of the melt 23 from the volume of the pulled single crystal 36 has been generally used. Since the amount of ascent of the crucible 21 is calculated by calculating the decrease in volume of the melt 23 from the volume of the pulled single crystal 36 in the above crucible ascent control, the accurate volume of the pulled single crystal 36 needs to be obtained. In addition, in order to calculate the accurate amount of the crucible ascent from the decrease in volume of the melt 23, the accurate internal diameter of the crucible is required. However, in actual pulls, errors are introduced to the crucible ascent control by errors in measuring crystal weight and crystal diameters, changes in the internal diameter of the quartz crucible 21a due to softening thereof during pulling, and changes in the internal diameter of the quartz crucible 21a between batches originating in variations in manufacturing the quartz crucible 21a. Therefore, it is necessary to actually measure to control the level position during pulling the single crystal 36.
In order to solve the above problems, a method for measuring the level position has been disclosed in Japanese Kokai No. 63-281022. In the method for measuring the level position, the level position is calculated from the position of the mirror image of a radiation screen reflected in the melt surface. The method makes it possible to measure the level position during crystal pulling. Similar to the method disclosed in the above publication, a method wherein the level position is controlled based on the mirror image position of a radiation screen reflected in the melt surface has been disclosed in Japanese Kokai No. 07-277879. In the method, by moving a crucible up and down so that the mirror image position of the radiation screen is kept fixed, the level position is controlled so as to be kept fixed.
However, in these methods, the inclination of the melt surface in which the mirror image of the radiation screen is reflected causes a change of the mirror image position even when the level position does not actually change, leading to the erroneous recognition and control of the level position. Factors of the inclination of the melt surface are changes in crystal diameter and changes in crucible rotational speed. Therefore, errors arise with changes in crystal diameter and changes in crucible rotational speed when the level position is controlled based on only the mirror image position of the radiation screen.
Particularly, the shoulder formation step wherein the crystal diameter is increased to a desired diameter from the seed crystal is a problem. In the shoulder formation step, the inclination of the melt surface increases with an increase in crystal diameter. Therefore, when the level position control by which the mirror image position of the radiation screen is kept fixed is conducted in the shoulder formation step, the actual level positions before and after the shoulder formation step become different.
The same errors are also caused by changes in crystal diameter in the main body formation step. In addition, when the level position control is conducted based on only the mirror image position of the radiation screen on the melt surface, the occurrence of irregularity in measuring the level position makes it impossible to carry out the ascent/descent control of the crucible 21.